Modifications of soil nitrogen pools in response to alfalfa root systems and shoot mulch

Decomposing alfalfa (Medicago sativa L.) shoots and roots generate large am ounts of NO3-N available to the next crop but also susceptible to deep leac hing. This study was aimed at determining the specific contributions of abo ve- and belowground alfalfa biomass to soil N pools. Dynamics of soil and p lant N pools were studied in a Kalamazoo loam soil (fine-loamy, mixed, mesi c Typic Hapludalfs) over a 2-yr period under bare fallow (BF), bare fallow to which alfalfa shoot mulch was applied (BFSM), living alfalfa plants with shoots removed after harvest (A), and living alfalfa with shoot mulch rema ining on the soil surface after harvest (ASM). Organic N pools were monitor ed in alfalfa plant parts, soil-incorporated debris, and soil organic matte r to depths of 150 cm. Inorganic N pools were monitored by suction lysimete rs, soil extraction, and evaluation of soil denitrification rates. Living a lfalfa stands kept soil inorganic N at very low levels, whether shoot mulch was applied or not. Soluble inorganic N concentrations decreased earlier i n the fall in the upper horizons of bare fallow soils receiving alfalfa sho ot mulch, suggesting enhanced leaching from bare soil under alfalfa mulch, Alfalfa crown and roots contained an average of 115 kg N ha(-1) after 2 yr of treatment, In conclusion, alfalfa shoot mulch contributed little to sust ained increases in soil N pools, while crowns and roots contributed larger quantities to the soil N pool.